The present invention disclosed herein relates to a silicon photomultiplier and a method for fabricating the same, and more particularly, to a high density and high efficiency silicon photomultiplier and a method for fabricating the same.
Silicon photomultipliers are similar in structure to avalanche photodiodes, but they are characterized by operating in the Geiger mode. Under the Geiger mode, the number of charge carriers generated by photoexcitation which is initiated by light incident on photomultipliers geometrically increases through impact ionization which occurs as a result of acceleration by internal electrostatic field. Furthermore, while the avalanche photodiode has low current-to-photon gain (<1000) in the linear mode, silicon photomultipliers have breakdown current without restrictions of multiplication gain in the Geiger mode. Accordingly, with a simple structure, silicon photomultipliers are capable of high sensitivity photodetection up to the level of single photon.
Silicon photomultipliers include a plurality of microcells each of which independently detects and amplifies photon signals. When photons enter into microcells to produce electron-hole pairs, the Geiger breakdown occurs to generate a predetermined signal of which a magnitude corresponds to the amount of charges accumulated in the reverse-biased microcell diode.
Microcells in silicon photomultipliers are typically comprised of multiple layers doped with p+/n−/n+ or n+/p−/p+. As a reverse voltage applied to the diode structure increases, a depletion layer between p+ and n+ regions expands so that a breakdown occurs at a voltage above the breakdown voltage.
On the other hand, when a breakdown occurs in a microcell, the microcell becomes unable to operate, and the device may be damaged by the large currents. Therefore, the microcell should be recovered from the breakdown state in a short period of time. For this, a passive quenching technique employing a series connection of a quench resistor to a diode of a microcell may be applied to silicon photomultipliers. When an instantaneous current flows by electrical breakdown, a quench resistor induces an ohmic voltage drop to protect the device and to clear the breakdown state.
Such silicon photomultipliers may be fabricated using a semiconductor photolithography. When photolithography is employed to form a silicon photomultiplier, processes of forming a diode and a quench resistor are carried out separately. Consequently, the cost of manufacturing a silicon photomultiplier increases.
Since the area onto which the quench resistors are formed cannot work as the light sensing area, the photon detection efficiency becomes lower as the quench resistor area becomes larger in a silicon photomultiplier. Consequently, a method for reducing the quench resistor forming region has been in need.
Further, since the dynamic range of light intensity measurement in silicon photomultipliers depends on the ratio of the light-sensing area of one microcell over the total surface area of the device and the number of microcells, a method for forming high-density microcells is required to improve the dynamic range.